Surface-coated cutting tool and method of manufacturing the same

ABSTRACT

A surface-coated cutting tool includes a base material and a coating formed on the base material. The coating includes an α-Al 2 O 3  layer. The α-Al 2 O 3  layer contains α-Al 2 O 3  crystal grains and sulfur, and has a TC(006) of more than 5 in texture coefficient TC(hkl). The sulfur has a concentration distribution in which a concentration of the sulfur decreases in a direction away from a base-material-side surface of the α-Al 2 O 3  layer, in a thickness direction of the α-Al 2 O 3  layer.

TECHNICAL FIELD

The present invention relates to a surface-coated cutting tool and amethod of manufacturing the same.

BACKGROUND ART

A surface-coated cutting tool having a coating formed on a base materialhas conventionally been used. Recently, various techniques have beenproposed for enhancing the performance of the surface-coated cuttingtool, such as a technique for improving the quality of the coating bychanging the crystallographic texture of Al₂O₃. For example, JapanesePatent Laying-Open No. 2008-246664 (PTD 1) proposes a cutting toolincluding an α-Al₂O₃ layer having the (006) texture on a base materialof a cemented carbide.

European Patent Publication No. 2570510 (PTD 2) proposes a cutting toolincluding an α-Al₂O₃ layer having the (0012) texture and containing 100ppm or more of sulfur on a base material of a cemented carbide.

CITATION LIST Patent Document PTD 1: Japanese Patent Laying-Open No.2008-246664 PTD 2: European Patent Publication No. 2570510 SUMMARY OFINVENTION Technical Problem

However, while the cutting tools disclosed in above-referenced PTD 1 andPTD 2 have increased wear resistance, the fracture resistance may beinsufficient or the effect of reducing the friction coefficient may notsufficiently be obtained.

The present invention has been made in view of the above-describedcircumstances, and an object of the present invention is to provide asurface-coated cutting tool which can achieve an extended life by itscoating formed to have both excellent wear resistance and excellentslidability, and to provide a method of manufacturing the surface-coatedcutting tool.

Solution to Problem

A surface-coated cutting tool according to an aspect of the presentinvention includes a base material and a coating formed on the basematerial. The coating includes an α-Al₂O₃ layer. The α-Al₂O₃ layercontains a plurality of α-Al₂O₃ crystal grains and sulfur, and has aTC(006) of more than 5 in texture coefficient TC(hkl). The sulfur has aconcentration distribution in which a concentration of the sulfurdecreases in a direction away from a base-material-side surface of theα-Al₂O₃ layer, in a thickness direction of the α-Al₂O₃ layer.

Advantageous Effects of Invention

According to the foregoing, both the excellent wear resistance and theexcellent slidability are exhibited, and the life can be extended.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photomicrograph as a substitute for a diagram, measurementpoints in an α-Al₂O₃ layer used for measuring the content of sulfur (S)by EDS being indicated on the photomicrograph.

DESCRIPTION OF EMBODIMENTS Description of Embodiment of the Invention

The inventors of the present invention have thoroughly conducted studiesfor solving the above-described problems, and finally reached thepresent invention. In a process of nucleation of an α-Al₂O₃ layer, alarge amount of H₂S was introduced in a pulsed form to thereby generatea sulfur concentration distribution in the thickness direction of theα-Al₂O₃ layer. Specifically, in the sulfur concentration distribution,the sulfur concentration decreases in the direction away from the basematerial in the thickness direction of the α-Al₂O₃ layer. It was foundthat excellent wear resistance and excellent slidability can thus beexhibited.

First of all, the present invention will be described based on featureslisted below.

[1] A surface-coated cutting tool according to an aspect of the presentinvention includes a base material and a coating formed on the basematerial. The coating includes an α-Al₂O₃ layer. The α-Al₂O₃ layercontains a plurality of α-Al₂O₃ crystal grains and sulfur, and has aTC(006) of more than 5 in texture coefficient TC(hkl). The sulfur has aconcentration distribution in which a concentration of the sulfurdecreases in a direction away from a base-material-side surface of theα-Al₂O₃ layer, in a thickness direction of the α-Al₂O₃ layer. Thesurface-coated cutting tool having the above-specified features canexhibit excellent wear resistance and excellent slidability.

[2] Preferably, the α-Al₂O₃ crystal grains having a grain size of 0.2 to2 μm occupy 20 to 80% by area of a measurement surface, the measurementsurface is in parallel with a surface of the α-Al₂O₃ layer or parallelwith an interface between the α-Al₂O₃ layer and an adjacent layer, theadjacent layer is adjacent to the α-Al₂O₃ layer and located on anopposite side to the base material, and the measurement surface islocated at a depth of 0.5 μm from the surface of the α-Al₂O₃ layer orthe interface. Accordingly, the wear resistance can be improved.

[3] Preferably, the TC(006) is more than 6. Accordingly, the wearresistance and the slidability of the tool are effectively improved.

[4] Preferably, the TC(006) is more than 7. Accordingly, the wearresistance and the slidability of the tool are more effectivelyimproved.

[5] Preferably, a maximum concentration Csmax of the sulfur in theconcentration distribution appears in a region of 1 μm from an interfacebetween the α-Al₂O₃ layer and the base material, or from an interfacebetween the α-Al₂O₃ layer and a layer adjacent to the α-Al₂O₃ layer andlocated on the same side as the base material, in the thicknessdirection of the α-Al₂O₃ layer; a minimum concentration Csmin of thesulfur in the concentration distribution appears in a region of 1 μmfrom a surface of the α-Al₂O₃ layer or from an interface between theα-Al₂O₃ layer and a layer adjacent to the α-Al₂O₃ layer and located onan opposite side to the base material, in the thickness direction of theα-Al₂O₃ layer; and the Csmax is 0.005 to 1 at. %, the Csmin is 0.001 to0.1 at. %, and the Csmax and the Csmin meet a relation Csmax>Csmin.Accordingly, the wear resistance can be improved.

[6] Preferably, a maximum concentration Csmax of the sulfur in theconcentration distribution is 0.005 to 1 at. %. Accordingly,particularly the slidability can be improved.

[7] Preferably, the α-Al₂O₃ layer has an average layer thickness of 1 to15 μm. Accordingly, both the wear resistance and the fracture resistancecan be achieved.

[8] Preferably, in a surface of the coating, an outermost surface layerin which any one of Ti carbide, Ti nitride, and Ti boride is a maincomponent is disposed. Accordingly, identification of the corner of thetool is facilitated.

[9] Preferably, the coating has an intermediate layer between theα-Al₂O₃ layer and the base material, and the intermediate layer containsacicular TiCNO or acicular TiBN and has an average layer thickness of0.3 to 1 μm, and a difference between a maximum thickness and a minimumthickness of the intermediate layer is 0.3 μm or more. Accordingly, theadhesion of the α-Al₂O₃ layer in the coating can be improved.

[10] A method of manufacturing a surface-coated cutting tool accordingto an aspect of the present invention includes the step of forming, onthe base material by a CVD method, the coating including the α-Al₂O₃layer. In the step, a content of H₂S gas contained in a raw material gasin an initial stage of formation of the α-Al₂O₃ layer is 0.5 to 5 vol %,and the content of H₂S gas is momentarily increased to 0.65 to 7 vol %.Accordingly, a surface-coated cutting tool that can exhibit excellentwear resistance and excellent slidability can be manufactured.

Details of Embodiment of the Invention

In the following, an embodiment of the present invention (hereinafteralso referred to as “present embodiment”) will be described in furtherdetail.

<Surface-Coated Cutting Tool>

A surface-coated cutting tool of the present embodiment includes a basematerial and a coating formed on the base material. The coatingpreferably covers the entire surface of the base material. However, eventhe cutting tool in which a part of the base material is not coveredwith this coating or the structure and makeup of the coating ispartially different does not go beyond the scope of the presentinvention.

The surface-coated cutting tool of the present embodiment can suitablybe used as a cutting tool such as drill, end mill, indexable insert forthe drill, indexable insert for the end mill, indexable insert formilling, indexable insert for turning, metal-slitting saw, gear-cuttingtool, reamer, tap, or the like.

<Base Material>

As the base material, any base material conventionally known as a basematerial of this type may be used. For example, the base material ispreferably any of a cemented carbide (including, for example, a WC-basedcemented carbide, a cemented carbide containing WC and Co, and acemented carbide containing WC and Co and additionally a carbonitride ofTi, Ta, Nb or the like), a cermet (having a main component such as TiC,TiN, TiCN or the like), a high-speed steel, a ceramic material (such astitanium carbide, silicon carbide, silicon nitride, aluminum nitride,aluminum oxide, or the like), cubic boron nitride sintered body, and adiamond sintered body.

Among these variety of base materials, the cemented carbide,particularly WC-based cemented carbide, or the cermet (particularlyTiCN-based cermet) is preferably selected. These base materials areparticularly excellent in balance between hardness and strength at hightemperature, and have excellent characteristics for the base material ofthe surface-coated cutting tool for the above-described use.

In the case where the surface-coated cutting tool is an indexable insertor the like, the base material may have or may not have a chip breaker.Moreover, the shape of the edge ridgeline may be any of a sharp edge(the ridge where the rake face and the flank face meet each other), ahoned edge (a sharp edge processed to be rounded), a negative land(beveled), and a combination of the honed edge and the negative land.

<Coating>

The coating includes an α-Al₂O₃ layer. For example, the coating may bemade up of a plurality of layers including at least one α-Al₂O₃ layerand further including other layers.

Examples of the aforementioned other layers may be TiCNO layer, TiBNlayer, TiC layer, TiN layer, TiAlN layer, TiSiN layer, AlCrN layer,TiAlSiN layer, TiAlNO layer, AlCrSiCN layer, TiCN layer, TiSiC layer,CrSiN layer, AlTiSiCO layer, TiSiCN layer, and the like. A compoundexpressed herein by a chemical formula like the above-referenced onesincludes the compound with any of all conventionally known atomic ratiosif the atomic ratio is not particularly limited, and the compound is notnecessarily limited to the compound with a stoichiometric ratio.

For example, in the case of an expression “TiAlN,” the ratio of thenumber of atoms between the elements constituting TiAlN is not limitedto Ti:Al:N=0.5:0.5:1, and includes all conventionally known ratios ofthe number of atoms. This is applied as well to any expressions ofcompounds other than “TiAlN.” Moreover, in the present embodiment, themetal element such as titanium (symbol: Ti), aluminum (symbol: Al),silicon (symbol: Si), zirconium (symbol: Zr), or chromium (symbol: Cr)and the non-metal element such as nitrogen (symbol: N), oxygen (symbol:O), or carbon (symbol: C) may not necessarily constitute astoichiometric composition.

The coating has an average thickness of 3 to 35 μm (3 μm or more and 35μm or less, it should be noted that a numerical value range expressedwith “-” or “to” herein includes the numerical values of the upper limitand the lower limit). Preferably, the coating has an average thicknessof 5 to 20 μm. If this average thickness is less than 3 μm, there is apossibility that the wear resistance is insufficient. If this averagethickness is more than 35 μm, there is a possibility that the coating ispeeled off or broken highly frequently when a large stress is appliedbetween the coating and the base material during intermittentprocessing.

<α-Al₂O₃ Layer>

The α-Al₂O₃ layer contains a plurality of α-Al₂O₃ (aluminum oxide havingan α-type crystal structure) crystal grains and sulfur (symbol: S). Theα-Al₂O₃ layer includes polycrystalline α-Al₂O₃ containing a plurality ofα-Al₂O₃ crystal grains. The crystal grains usually have a grain size onthe order of 0.1 to 2 μm.

In the α-Al₂O₃ layer, many α-Al₂O₃ crystal grains are in the (006)orientation. The α-Al₂O₃ layer has a TC(006) of more than 5 in texturecoefficient TC(hkl) expressed by the following expression (1).

$\begin{matrix}{{{TC}({hkl})} = {\frac{I({hkl})}{I_{0}({hkl})}\left\{ {\frac{1}{n}{\sum_{1}^{n}\frac{I({hkl})}{I_{0}({hkl})}}} \right\}^{- 1}}} & (1)\end{matrix}$

In the expression (1), I(hkl) represents an x-ray diffraction intensityof a (hkl) reflection plane, and I₀(hkl) represents a standard intensityaccording to PDF card No. 00-010-0173 of the ICDD. In the expression(1), n represents the number of reflections used for the calculation andis eight in the present embodiment. (hkl) planes used for reflection are(012), (104), (110), (006), (113), (024), (116), and (300).

ICDD (registered trademark) is an abbreviation for International Centerof Diffraction Data. PDF (registered trademark) is an abbreviation forPower Diffraction File.

TC(006) of the α-Al₂O₃ layer in the present embodiment can be expressedby the following expression (2).

$\begin{matrix}{{{TC}(006)} = {\frac{I(006)}{I_{0}(006)}\left\{ {\frac{1}{8}{\sum_{1}^{8}\frac{I({hkl})}{I_{0}({hkl})}}} \right\}^{- 1}}} & (2)\end{matrix}$

“TC(006) of more than 5 in texture coefficient TC(hkl)” means that anumerical value given by the above expression (2) which is determined bysubstituting TC(006) in the expression (1) is more than 5. The α-Al₂O₃layer having a TC(006) of more than 5 has effective hardness and Young'smodulus against impact and vibration under severe cutting conditions,and therefore can contribute to improvement of the wear resistance.

The value of TC(006) is preferably more than 6 and more preferably morethan 7. A greater value of TC(006) enables effective improvement of thewear resistance. While the upper limit of the value of TC(006) is notlimited, the upper limit may be 8 or less since the number of reflectionplanes used for the calculation is 8.

This TC(hkl) can be measured through analysis by means of an x-raydiffractometer. TC(hkl) can for example be measured by means of SmartLab(registered trademark) manufactured by Rigaku Corporation (scanningspeed: 21.7°/min, step: 0.01°, scanning range: 15 to 140°) under thefollowing conditions. It should be noted that the result of measurementof the TC(hkl) by means of the x-ray diffractometer is herein referredto as “XRD result.”

characteristic x-ray: Cu-Kα

tube voltage: 45 kV

tube current: 200 mA

filter: multilayer mirror

optical system: focusing method

x-ray diffraction method: θ-2θ method

<Concentration Distribution of Sulfur Contained in α-Al₂O₃ Layer>

Sulfur (in some cases referred to hereinafter as “S” which is the atomicsymbol of sulfur) contained in the α-Al₂O₃ layer has a concentrationdistribution in which the concentration of sulfur decreases in thedirection away from a base-material-side surface of the α-Al₂O₃ layer inthe thickness direction of the α-Al₂O₃ layer. Specifically, in the casefor example where point A, point B, and point C are set in this order inthe direction away from the base-material-side surface of the α-Al₂O₃layer and the S content at each of these points is measured, theconcentration distribution meets the relation “S content at point A>Scontent at point B>S content at point C.” Such a form of theconcentration distribution of S enables excellent wear resistance to beachieved and enables significant improvement of the slidability.

Here, sulfur has “a concentration distribution in which theconcentration of sulfur decreases in the direction away from abase-material-side surface of the α-Al₂O₃ layer in the thicknessdirection of the α-Al₂O₃ layer” means that the α-Al₂O₃ layer may includea portion where the concentration of sulfur decreases in the directionaway from the base-material-side surface of the α-Al₂O₃ layer in thethickness direction of the α-Al₂O₃ layer. It also means that a relation“S content at point X>S content at point Y” is always met, where point Xis a point directly above the interface between the α-Al₂O₃ layer andthe base material (in the case where an adjacent layer is locatedbetween the α-Al₂O₃ layer and the base material, a point immediatelyabove the interface between the α-Al₂O₃ layer and the adjacent layer),and point Y is a point immediately below the surface of the α-Al₂O₃layer (in the case where another adjacent layer is located on theα-Al₂O₃ layer oppositely to the base material, a point immediately belowthe interface between the α-Al₂O₃ layer and this adjacent layer).

As long as the α-Al₂O₃ layer includes the portion where theconcentration of sulfur decreases in the direction away from thebase-material-side surface of the α-Al₂O₃ layer and the relation “Scontent at point X>S content at point Y” is met, the α-Al₂O₃ layer mayinclude a portion where the concentration of S is constant in thedirection away from the base-material-side surface of the α-Al₂O₃ layer.Moreover, the α-Al₂O₃ layer may include a portion where theconcentration of S increases in the direction away from thebase-material-side surface of the α-Al₂O₃ layer or a portion where nosulfur is contained, for example.

Preferably, a maximum concentration Csmax of sulfur in the concentrationdistribution appears in a region of 1 μm from an interface between theα-Al₂O₃ layer and the base material, or from an interface between theα-Al₂O₃ layer and a layer adjacent to the α-Al₂O₃ layer and located onthe same side as the base material, in the thickness direction of theα-Al₂O₃ layer. Preferably, a minimum concentration Csmin of the sulfurin the concentration distribution appears in a region of 1 μm from asurface of the α-Al₂O₃ layer or from an interface between the α-Al₂O₃layer and a layer adjacent to the α-Al₂O₃ layer and located on anopposite side to the base material, in the thickness direction of theα-Al₂O₃ layer. Preferably, Csmax is 0.005 to 1 at. %, the Csmin is 0.001to 0.1 at. %, and the Csmax and the Csmin meet a relation Csmax>Csmin.Accordingly, the wear resistance can be improved. For example, Csmax mayappear at a location immediately above the interface between the α-Al₂O₃layer and the base material or a location directly above an adjacentlayer if the adjacent layer is located between the α-Al₂O₃ layer and thebase material. Csmin may appear at a location immediately below thesurface of the α-Al₂O₃ layer or a location immediately below anotheradjacent layer if this adjacent layer is located on the α-Al₂O₃ layeroppositely to the base material.

More preferably, the difference between Csmax and Csmin is 0.1 at. % ormore. The upper limit of the difference between Csmax and Csmin may be0.9 at. %. If the difference is larger than this, there is a possibilitythat coarse crystal grains may be generated. Moreover, Csmax ispreferably 0.005 to 1 at. %, more preferably 0.05 to 1 at. %, and stillmore preferably 0.1 to 0.7 at. %. Accordingly, the slidability can beimproved. If Csmax is less than 0.005 at. %, the slidability isinsufficient and there is a possibility that adhesion of the workpieceis likely to increase during cutting. If Csmax is more than 1 at. %,there is a possibility that the fracture resistance is deteriorated.

Preferably, Csmin is 0.001 to 0.01 at. %. In the case where Csmin isless than 0.001 at. %, further enhancement of the wear resistance isdifficult to achieve.

In the present embodiment, the content of S in the α-Al₂O₃ layer isexpressed by at. %. Specifically, the S content may be expressed by anatomic composition percentage [S/(Al+O+C+Cl+Ti+S)×100] where thedenominator is the sum of respective numbers of atoms of Al, 0, C,chlorine (symbol: Cl), Ti, and S, and the numerator is the number ofatoms of S.

The S content can be measured in the following way. Polishing by ionmilling is performed on a coating cross section extending in parallelwith a cross section of the α-Al₂O₃ layer in the thickness directionthereof, and the polished cross section is analyzed by means of anenergy-dispersive x-ray spectroscopy (EDS) analyzer using a fieldemission scanning electron microscope. Moreover, the S content can bemeasured in further detail by means of the WDS (Wavelength DispersiveX-ray Spectroscopy) analysis method.

-   -   The conditions of the aforementioned ion milling are for example        as follows.    -   acceleration voltage: 6 kV    -   ion beam incident angle: 0 to 5° from the normal line    -   ion beam irradiation time: 300 minutes

In the present embodiment, the S content was measured by means of an EDSanalyzer using a field emission scanning electron microscope of SU6600(model No.) manufactured by Hitachi High-Technologies Corporation. Formeasurement, the acceleration voltage of the field emission scanningelectron microscope was set to 15 kV. The conditions of EDS were set sothat the number of frames was 150 and selected elements were C, O, Al,S, Cl, Ti. As shown in FIG. 1, the S content in an α-Al₂O₃ layer 1 wasmeasured at each of points located at predetermined intervals in thethickness direction, from the interface (TiCNO layer 3) between α-Al₂O₃layer 1 and a layer (TiCN layer 2) which is located between the α-Al₂O₃layer and the base material, toward the surface of the coating, and thedistribution of the content was analyzed.

In FIG. 1, the measurement points in α-Al₂O₃ layer that were used formeasuring the S content by EDS are indicated on a photomicrograph. InFIG. 1, the coating includes TiCN layer 2 formed on the base material,TiCNO layer 3 deposited on this TiCN layer 2, and α-Al₂O₃ layer 1deposited on TiCNO layer 3. For this analysis, measurement points 4(first measurement point 41, second measurement point 42, thirdmeasurement point 43, fourth measurement point 44, fifth measurementpoint 45) were set from the position directly above TiCNO layer 3 whichis the interface between α-Al₂O₃ layer 1 and TiCN layer 2, atpredetermined intervals (at intervals of 1.0 μm for example), in thethickness direction toward the surface of the coating. Then, the Scontent was measurement at each of the measurement points from firstmeasurement point 41 to fifth measurement point 45.

<Grain Size of α-Al₂O₃ Crystal Grains Contained in α-Al₂O₃ Layer>

Preferably, in the α-Al₂O₃ layer, the α-Al₂O₃ crystal grains having agrain size of 0.2 to 2 μm occupy 20 to 80% by area of a measurementsurface which is parallel with a surface of the α-Al₂O₃ layer orparallel with an interface between the α-Al₂O₃ layer and an adjacentlayer, the adjacent layer is adjacent to the α-Al₂O₃ layer and locatedon an opposite side to the base material, and the measurement surface islocated at a depth of 0.5 μm from the surface or the interface. If theα-Al₂O₃ crystal grains occupying 20 to 80% by area of the measurementsurface have a grain size of less than 0.2 μm, there is a possibilitythat the fracture resistance is deteriorated. If the grain size is morethan 2 μm, there is a possibility that the wear resistance isdeteriorated.

The upper limit of the grain size is preferably 1.85 μm. The lower limitof the grain size is 0.2 μm. As long as the value of the grain size isnot less than 0.2 μm, the value is a preferable value. This is for thereasons that the fracture resistance can be improved simultaneously withthe wear resistance as long as the grain size falls in the range asdefined above.

Moreover, it is not preferable that the ratio of α-Al₂O₃ crystal grainswith a grain size of 0.2 to 2 μm is less than 20% by area or more than80% by area of the measurement surface, since the fracture resistanceand the wear resistance cannot be improved in this case. A morepreferred ratio of the α-Al₂O₃ crystal grains with a grain size of 0.2to 2 μm is 50 to 70% by area.

In the present embodiment, measurement of the grain size of the α-Al₂O₃crystal grains in the measurement surface of the α-Al₂O₃ layer wasperformed on a coating cross section extending in parallel with a crosssection of the α-Al₂O₃ layer in the thickness direction thereof.Specifically, in this coating cross section, a location at 0.5 μm awayfrom the surface of the α-Al₂O₃ layer (from the interface between theα-Al₂O₃ layer and an adjacent layer if the adjacent layer is located onthe α-Al₂O₃ layer oppositely to the base material) toward the inside ofthe α-Al₂O₃ layer was observed with a field emission scanning electronmicroscope. The grain size of the α-Al₂O₃ crystal grains can be measuredfrom a photomicrograph image thereof by means of the section method. Thecoating cross section may be polished by ion milling, a photomicrographimage of the polished surface may be used to conduct EBSD analysis, andthe grain size of the α-Al₂O₃ crystal grains may be measured by thisanalysis.

As the section method used for measurement of the grain size of theα-Al₂O₃ crystal grains, a method was employed according to which thenumber of crystal grains across a certain width was counted and thewidth was divided by the counted number of crystal grains to therebycalculate the grain size.

<Thickness of α-Al₂O₃ Layer>

The α-Al₂O₃ layer preferably has an average thickness of 1 to 15 μm.Accordingly, both the wear resistance and the fracture resistance can beachieved. If the average thickness of the α-Al₂O₃ layer is less than 1μm, there is a possibility that wear is likely to increase. If theaverage thickness is more than 15 μm, there is a possibility that thefracture resistance is deteriorated.

<Other Layers>

The coating may include layers other than the α-Al₂O₃ layer as describedabove. An example of the other layers is a TiCN layer for example. ThisTiCN layer is excellent in wear resistance and therefore can providehigher wear resistance to the coating. The TiCN layer is particularlypreferably formed by the MT-CVD (medium temperature CVD) method. Amongthe CVD methods, the MT-CVD method can be used to form a layer at arelatively low temperature of approximately 800 to 1000° C., and canreduce damage to the base material caused by heating in the process offorming the layer. The TiCN layer may be disposed for example betweenthe α-Al₂O₃ layer and the base material.

The average thickness of the TiCN layer is preferably 2 to 20 μm. Ifthis average thickness is less than 2 μm, there is a possibility thatwear is likely to increase. If this average thickness is more than 20μm, there is a possibility that the fracture resistance is deteriorated.

It should be noted that an outermost surface layer and an intermediatelayer described below are also included in the other layers.

<Outermost Surface Layer>

Preferably, in a surface of the coating, an outermost surface layer inwhich any one of Ti carbide, Ti nitride, and Ti boride is a maincomponent is disposed. The outermost surface layer is a layer located atthe outermost surface position in the coating. It should be noted,however, that the outermost surface layer may not be formed in a regionincluding the edge ridgeline. In the case where other layers are notformed on the α-Al₂O₃ layer, the outermost surface layer is disposeddirectly on the α-Al₂O₃ layer.

“Any one of Ti carbide, Ti nitride, and Ti boride is a main component”means that the outermost surface layer contains 90 mass % or more of anyone of Ti carbide, Ti nitride, and Ti boride. It preferably means thatthe outermost surface layer is made of any one of Ti carbide, Tinitride, and Ti boride, besides inevitable impurities.

In the case where the outermost surface layer is formed, the outermostsurface layer has an effect of assuming a clear chromatic color forexample to thereby make it easy to identify a corner (identify a usedpart) of a cutting insert after used for cutting.

The outermost surface layer preferably has an average thickness of 0.05to 1 μm. The upper limit of the average thickness of the outermostsurface layer is preferably 0.8 μm and more preferably 0.6 μm. The lowerlimit of the average thickness of the outermost surface layer ispreferably 0.1 μm and more preferably 0.2 μm. If the average thicknessis less than 0.05 μm, there is a possibility that the effect ofimproving the fracture resistance is not sufficiently provided when acompressive residual stress is applied to the coating, and the fractureresistance is not improved. If the average thickness is more than 1 μm,there is a possibility that the adhesion between the outermost surfacelayer and a layer adjacent to the outermost surface layer isdeteriorated.

<Intermediate Layer>

Preferably, the coating has an intermediate layer between the α-Al₂O₃layer and the base material. The intermediate layer is formed to containacicular TiCNO or acicular TiBN in acicular shape. For example, theintermediate layer is preferably disposed between the α-Al₂O₃ layer anda TiCN layer which is disposed between the α-Al₂O₃ layer and the basematerial, and more preferably disposed between the α-Al₂O₃ layer and theTiCN layer and in contact with both the α-Al₂O₃ layer and the TiCNlayer, since the adhesion of the α-Al₂O₃ layer in the coating isincreased. The intermediate layer may be formed by any known method.

The intermediate layer preferably has an average thickness of 0.3 to 1μm, since the adhesion of the α-Al₂O₃ layer in the coating is furtherincreased. The average thickness of the intermediate layer is morepreferably 0.4 to 0.8 μm. Further, a difference between a maximumthickness and a minimum thickness of the intermediate layer ispreferably 0.3 μm or more. Accordingly, increase of the adhesion of theα-Al₂O₃ layer in the coating can be ensured. If the difference betweenthe maximum thickness and the minimum thickness of the intermediatelayer is less than 0.3 μm, there is a possibility that the effect ofincreasing the adhesion of the α-Al₂O₃ layer is not sufficientlyobtained. The upper limit of the difference between the maximumthickness and the minimum thickness of the intermediate layer is 0.9 μm.If the difference is more than 0.9 μm, there is a possibility that theα-Al₂O₃ crystal grains are non-uniform and the adhesion is deteriorated.

The method of measuring the thickness of the intermediate layer issimilar to the method used for measuring the grain size of the α-Al₂O₃crystal grains. Namely, a coating cross section extending in parallelwith a cross section of the α-Al₂O₃ layer in the thickness directionthereof is observed with a field emission scanning electron microscopeto measure the thickness of the intermediate layer. The averagethickness of the intermediate layer can be determined for example in thefollowing way. From a plurality of locations of the intermediate layerat which the thickness thereof is measured by the above-describedmeasurement method, any several thicknesses are selected, and theaverage of the thicknesses at the several locations is calculated. Themaximum thickness and the minimum thickness of the intermediate layercan also be determined by taking the maximum thickness and the minimumthickness from thicknesses at multiple locations of the intermediatelayer measured by the above-described measurement method. Moreover, themeasurement can also be conducted by polishing the cross section by ionmilling and using a microphotograph image of the polished cross section.

<Method of Manufacturing Coating>

A method of manufacturing a surface-coated cutting tool in the presentembodiment includes the step of forming, on the base material by a CVDmethod, the coating including the α-Al₂O₃ layer. In the step, a contentof H₂S gas contained in a raw material gas in an initial stage offormation of the α-Al₂O₃ layer is 0.5 to 5 vol %, and the content of H₂Sgas is momentarily increased to 0.65 to 7 vol %. Here, the initial stageof formation of the α-Al₂O₃ layer is a stage in which nucleation ofα-Al₂O₃ crystal grains is done.

The surface-coated cutting tool can appropriately be manufactured byforming a coating on a base material by the chemical vapor deposition(CVD) method. In the case where the CVD method is used, the depositiontemperature is 800 to 1200° C. which is higher than the temperature forthe physical vapor deposition method and thus the adhesion between thecoating and the base material is improved. In the case where layersother than the α-Al₂O₃ layer are formed as layers of the coating, theselayers may be formed by a conventionally known method. Moreover, thethickness of the α-Al₂O₃ layer and respective thicknesses of the otherlayers can be adjusted by appropriately regulating the deposition time(the deposition rate for each layer is about 0.5 to 2.0 μm/hour).

The α-Al₂O₃ layer can be formed in the following manner by means of theCVD method for example.

First, by a known method, a TiCN layer is formed on another layer formedon the base material or a TiCN layer is formed on the base materialwithout another layer interposed therebetween, and a TiCNO layer isformed on a surface of the TiCN layer. Further, a surface of the TiCNOlayer is oxidized to cause nucleation of α-Al₂O₃ crystal grains.Subsequently, an α-Al₂O₃ layer is formed (α-Al₂O₃ crystal is grown). Fornucleation of α-Al₂O₃ crystal grains and formation of the α-Al₂O₃ layer(growth of α-Al₂O₃ crystal), the content of H₂S gas contained in a rawmaterial gas to be introduced is set to a content selected from a rangeof 0.5 to 5 vol %. The content of each gas in the raw material gas otherthan the H₂S gas is 1.3 to 2.5 vol % of AlCl₃, 2.8 to 6 vol % of HCl, 1to 5 vol % of CO, 0.4 to 3 vol % of CO₂, 0.002 to 0.008 vol % of O₂, andthe remainder of H₂. The temperature in a furnace of a CVD apparatus is970 to 1020° C. and the pressure in the furnace is 70 to 110 hPa.

It should be noted that the surface of the TiCNO layer is oxidized byCO, CO₂ and O₂ in the composition of the raw material gas for nucleationof α-Al₂O₃ crystal grains.

During nucleation of the α-Al₂O₃ crystal grains, the raw material gas isintroduced in which the content of H₂S gas contained in the raw materialgas is increased momentarily (in pulsed manner) from the contentselected from a range of 0.5 to 5 vol %. This operation can be performedonce or performed twice or more. Namely, firstly the content of H₂S gasis changed in pulsed manner to cause nucleation of the α-Al₂O₃ crystalgrains, and thereafter the raw material gas having the above-describedcomposition is used to grow the α-Al₂O₃ crystal grains and form theα-Al₂O₃ layer. In this way, the α-Al₂O₃ layer can be formed having aconcentration distribution of S in which the concentration of Sdecreases in the direction away from the base-material-side surface ofthe α-Al₂O₃ layer in the thickness direction. It should be noted in thecase where the content of H₂S gas is momentarily increased duringnucleation, the increase may be adjusted by decreasing the content of H₂gas which is the remainder of the raw material gas. This is convenientsince respective contents of the other gases, the temperature in thefurnace of the CVD apparatus, and the pressure in the furnace can bekept constant.

Preferably, the momentarily increased content (pulse height) of H₂S gasis 130 to 160% relative to the content selected from a range of 0.5 to 5vol %. If this is less than 130%, there is a possibility that Csmax is0.005 at. % or less. If this is more than 160%, there is a possibilitythat the crystal grains are coarsened and excellent wear resistancecannot be obtained.

It has conventionally been pointed out that an excessively increasedconcentration of introduced H₂S gas causes reaction of α-Al₂O₃ crystalgrains before formed on the base material, the crystal grains arecoarsened, and abnormal growth of the coating is caused. In the case ofthe surface-coated cutting tool in the present embodiment, abnormalgrowth of the α-Al₂O₃ crystal grains can be suppressed by introducingthe H₂S gas so that the concentration is changed in pulsed manner evenwhen the concentration of the introduced H₂S gas is high. As a result,the concentration of sulfur in the α-Al₂O₃ layer can efficiently beincreased and the α-Al₂O₃ layer having a concentration distribution of Sin which the concentration of S decreases in the direction away from thebase-material-side surface of the α-Al₂O₃ layer in the thicknessdirection thereof can be formed. As a result of evaluation of theperformance of the α-Al₂O₃ layer having such a concentrationdistribution of S, it is seen that the effect that the α-Al₂O₃ layer hasexcellent wear resistance and excellent slidability as will be describedlater herein can be obtained. In order to more effectively obtain theexcellent wear resistance and the excellent slidability, O₂ may be addedin addition to CO₂ as oxygen source of the raw material gas. This is forthe reason that addition of O₂ enables high (006) orientation to beobtained more effectively.

Moreover, H₂S gas contained in the raw material gas may be replaced withgas which is a combination of SO₂ gas and H₂S gas, a combination of SF₆gas and H₂S gas, or a combination of SO₂ gas and SF₆ gas with H₂S gas.In this case as well, the concentration distribution of S can be formedin the thickness direction of the α-Al₂O₃ layer.

EXAMPLES

In the following, the present invention will be described in furtherdetail with reference to Examples. The present invention, however, isnot limited to them.

Example 1

<Preparation of Base Material>

A base material formed of a cemented carbide base material (manufacturedby Sumitomo Electric Industries, Ltd.) with a shape of CNMG120408defined under JIS (Japanese Industrial Standard) B 4120 (1998) wasprepared. Prepared base materials were grouped into seven groups namedSample A1 to Sample A7. For each group, six base materials wereprepared. These base materials had a composition made up of 87.0 wt % ofWC, 8.0 wt % of Co, 2.5 wt % of TiC, 1.5 wt % of NbC, and 1.0 wt % ofTaC.

As will be described later herein, Sample A1 to Sample A4 are Examplesand Sample A5 to Sample A7 are Comparative Examples.

<Formation of Coating>

The base materials of Sample A1 to Sample A7 were subjected to honing bya known method and set in a chemical vapor deposition apparatus, and acoating was formed on a surface of each base material by the CVD method.Regarding the conditions for forming the coating, the conditions forforming each layer except for the α-Al₂O₃ layer are indicated in thefollowing Table 1.

TABLE 1 conditions for forming layer total temper- pres- gas compositionof raw material ature sure amount gas (vol %) (° C.) (hPa) (L/min) TiNTiCl₄ = 2%, N₂ = 25%, 900 200 60 (underlayer) H₂ = remainder TiCN TiCl₄= 2%, CH₃CN = 0.5%, 850 80 95 N₂ = 20%, H₂ = remainder TiCNO TiCl₄ = 1%,CO = 1%, 1000 250 60 CH₄ = 5%, N₂ = 10%, H₂ = remainder TiC TiCl₄ = 2%,CH₄ = 7%, 1000 500 60 H₂ = remainder TiN TiCl₄ = 1.5%, N₂ = 40%, 1000800 90 (outermost H₂ = remainder surface layer)

In the process of forming the α-Al₂O₃ layer by the CVD method, thesurface of the TiCNO layer formed on the surface of the TiCN layer wasoxidized to cause nucleation of α-Al₂O₃ crystal grains, and subsequentlythe α-Al₂O₃ layer was formed. In particular, for nucleation of α-Al₂O₃crystal grains and subsequent formation of the α-Al₂O₃ layer, thecontent of H₂S gas in the raw material gas to be introduced was set to0.6 vol %. The content of each gas in the composition including the H₂Sgas of the raw material gas is indicated in Table 2 below.

It should be noted that, for nucleation of α-Al₂O₃ crystal grains, thecontent of H₂S gas which was originally set to 0.6 vol % was momentarilymade higher than 0.6 vol % and this H₂S gas was introduced. After this,the content of H₂S gas was set to 0.6 vol % and the raw material gas wasused to grow α-Al₂O₃ crystal grains and form the α-Al₂O₃ layer.Respective contents of the constituent gases including H₂S in thecomposition of the raw material gas are indicated in Table 2 below.

Particularly in Example 1, the momentarily increased content (pulseheight) of H₂S gas introduced for Sample A1 to Sample A7 and the time(pulse width) for which the H₂S gas at the momentarily increased contentis introduced were varied. Specifically, for Sample A1 to Sample A4, thecontent 0.6 vol % of H₂S gas was set to the 150% pulse height (namely0.9 vol %) and the pulse width was varied in a range of 0.8 to 1.3minutes to introduce the H₂S gas. In contrast, for Sample A5 and A6, thecontent of introduced H₂S gas was not momentarily increased. For SampleA7, the content 0.6 vol % of H₂S gas was set to the 130% pulse height(namely 0.78 vol %) and the pulse width was set to 1.0 minute tointroduce the H₂S gas. The number of times the H₂S gas was introduced atthe momentarily increased content is three. These conditions are thoseindicated in the following Table 2.

TABLE 2 pulse height temperature pressure normal and pulse pulse Samplein furnace in furnace H₂ CO CO₂ O₂ AlCl₃ HCl H₂S number width period No.(° C.) (hPa) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %) oftimes (min) (min) Example A1 1000 75 remainder 2 0.5 0.005 2 3 0.6 150%,1.0 5 3 times A2 1000 75 remainder 2 0.5 0.005 2 3 0.6 150%, 0.8 5 3times A3 1000 75 remainder 2 0.5 0.005 2 3 0.6 150%, 1.2 5 3 times A41000 75 remainder 2 0.5 0.005 2 3 0.6 150%, 1.3 5 3 times Comparative A51000 75 remainder 2 0.5 0.005 2 3 1.2 — — — Example A6 1000 75 remainder2 0.5 0.005 2 3 0.6 — — — A7 1000 75 remainder 2 0.5 0.005 2 3 0.6 130%,1.0 5 3 times

The layer structure of the coating formed for each of Sample A1 toSample A7 is made up of a TiN layer, a TiCN layer, a TiCNO layer, anα-Al₂O₃ layer, a TiC layer, and a TiN layer in order from the basematerial. In the following Table 3, the layer structure of Sample A1 toSample A7 and the thickness (μm) of each layer are indicated.

TABLE 3 Sample No. layer structure and layer thickness (μm) Example A1base material/TiN(0.3)/TiCN(8.2)/TiCNO(1.0)/Al₂O₃(4.2)/TiC(0.3)/TiN(0.5) A2 basematerial/TiN(0.2)/TiCN(8.5)/TiCNO(1.0)/ Al₂O₃(4.4)/TiC(0.3)/TiN(0.4) A3base material/TiN(0.3)/TiCN(8.2)/TiCNO(1.0)/Al₂O₃(4.2)/TiC(0.3)/TiN(0.5) A4 basematerial/TiN(0.3)/TiCN(8.1)/TiCNO(1.0)/ Al₂O₃(4.1)/TiC(0.2)/TiN(0.5)Comparative A5 base material/TiN(0.3)/TiCN(8.2)/TiCNO(1.0)/ ExampleAl₂O₃(4.2)/TiC(0.3)/TiN(0.5) A6 basematerial/TiN(0.3)/TiCN(8.2)/TiCNO(1.0)/ Al₂O₃(4.2)/TiC(0.3)/TiN(0.5) A7base material/TiN(0.4)/TiCN(8.3)/TiCNO(1.2)/Al₂O₃(4.2)/TiC(0.3)/TiN(0.3)

<Test Details>

In Example 1, as described above, six samples were prepared for each ofSample A1 to Sample A7. For the first sample out of the six samples, theflank face was irradiated with x-ray to measure the TC(006) of theα-Al₂O₃ layer. For the second sample, the S content of the α-Al₂O₃ layerwas measured. For the third sample, the grain size of α-Al₂O₃ crystalgrains was measured. For the fourth sample, the slidability wasevaluated. For the fifth sample, the wear resistance was evaluated. Forthe sixth sample, the fracture resistance was evaluated.

The S content of the α-Al₂O₃ layer was measured at measurement points(first measurement point to fifth measurement point). The measurementpoints were set at intervals of 1 μm, from the position directly on theinterface between the α-Al₂O₃ layer and the TiCNO layer adjacent to thebase material side surface of the α-Al₂O₃ layer, toward the surface ofthe coating (see FIG. 1). The grain size of α-Al₂O₃ crystal grains wasmeasured at 0.5 μm away from the interface between the α-Al₂O₃ layer andthe TiC layer adjacent to the α-Al₂O₃ layer oppositely to the basematerial.

The methods of evaluating the wear resistance, the fracture resistance,and the slidability are as follows. The results of the evaluation areindicated in Table 4 below.

<Evaluation of Wear Resistance>

Workpiece: SCM435 (JIS)

Cutting Speed: 300 m/min

Feed: 0.3 mm/rev

Depth of Cut: 2.0 mm

Cutting Oil: dry

Cutting Time: 15 min

Evaluation: The wear width Vb (mm) of the flank face after cutting wasperformed for 15 minutes was measured.

The wear resistance is evaluated as follows. A cutting tool is set on anNC lathe, cutting of a workpiece is performed with the cutting tool fora predetermined time. After this, a wear width (Vb) formed on the flankface of the cutting tool is observed to evaluate the wear resistance. Acutting tool with a smaller value of the wear width (Vb) can beevaluated as being higher in wear resistance.

<Evaluation of Fracture Resistance>

Workpiece: SCM435 (JIS), grooved material

Cutting Speed: 200 m/min

Feed: 0.3 mm/rev

Depth of Cut: 1.5 mm

Cutting Oil: wet

Evaluation: The elapsed time (minutes) before chipping or fracture wasmeasured.

The fracture resistance is evaluated as follows. A cutting tool is seton an NC lathe, cutting of a workpiece is performed with the cuttingtool, and the elapsed time before chipping or fracture occurs to thecutting tool is measured to evaluate the fracture resistance. Therefore,a cutting tool with a longer elapsed time before chipping or fracturecan be evaluated as being higher in fracture resistance.

<Evaluation of Slidability>

The slidability was evaluated by measuring the friction coefficient (μ).The friction coefficient (μ) was measured by the pin-on-disk methodunder conditions of a 10 N load and room temperature. A cutting toolwith a smaller value of the friction coefficient (μ) has highersmoothness and can be evaluated as being higher in slidability.

TABLE 4 slida- S content (at. %) bility cutting performance 1st 2nd 3rd4th 5th fric- wear time mea- mea- mea- mea- mea- tion resis- to XRDsure- sure- sure- sure- sure- coeffi- grain tance frac- perfor- Sampleresult ment ment ment ment ment Csmax Csmax − cient size Vb ture manceNo. TC(006) point point point point point (at. %) Csmin μ μm (mm) (min)rating Example A1 6.77 0.082 0.020 0.012 0.008 0.003 0.082 0.079 0.350.93 0.161 6.5 A A2 5.71 0.050 0.012 0.007 0.005 0.003 0.050 0.047 0.381.85 0.168 5.9 B A3 7.79 0.220 0.051 0.005 0.006 0.005 0.220 0.215 0.330.53 0.154 7.7 AA A4 7.89 0.330 0.024 0.006 0.006 0.005 0.330 0.325 0.330.51 0.150 7.8 AA Comparative A5 3.25 0.761 0.091 0.049 0.009 0.0060.761 0.755 0.87 3.21 0.350 2.2 C Example A6 3.52 0.003 0.002 0.0010.000 0.000 0.003 0.003 0.57 2.10 0.235 5.2 D A7 4.68 0.002 0.002 0.0010.000 0.000 0.002 0.002 0.59 1.92 0.240 5.6 C

The rating of evaluation represented for example by symbol AA in Table 4is defined as follows.

AA: highly excellent in wear resistance, fracture resistance, andslidability (Vb of 0.165 or less, time to fracture of 6 minutes or more,and fracture coefficient (μ) of 0.35 or less)

A: excellent in wear resistance, fracture resistance, and slidability(two of the requirements, namely Vb of 0.165 or less, time to fractureof 6 minutes or more, and fracture coefficient (μ) of 0.35 or less aresatisfied)

B: sufficient in required wear resistance, fracture resistance, andslidability (Vb of 0.165 to 0.170, time to fracture of 4 to 6 minutes,and fracture coefficient (μ) of 0.35 to 0.54)

C: insufficient in wear resistance, fracture resistance, and slidability(Vb of 0.170 to 0.180 or time to fracture of 4 to 6 minutes, andfracture coefficient (μ) of more than 0.54)

D: impossible (evaluation is impossible due to occurrence of coarsegrains)

<Results of Evaluation>

As seen from Table 4, Examples corresponding to Sample A1 to Sample A4exhibit the performance that Vb (mm) is 0.170 or less and theperformance that the time to fracture is 5 minutes or more and thus canbe evaluated as having excellent wear resistance and fractureresistance. As to the evaluation of slidability, the frictioncoefficient (μ) of the Examples is 0.54 or less, and it has been madeclear that particularly the Examples with a friction coefficient of 0.38or less have sufficient slidability.

As to the XRD results in Table 4, the Examples have a TC(006) of theα-Al₂O₃ layer of more than 5. As to the S content, a Csmax of 0.05 to0.330 at. % is obtained at the first measurement point included in theα-Al₂O₃ layer and located at a side of the TiCNO layer. In the thicknessdirection of the α-Al₂O₃ layer, the S content decreases in the directionaway from the TiCNO layer, and a Csmin of 0.003 to 0.005 at. % isobtained at the fifth measurement point included in the α-Al₂O₃ layerand located at a side of the TiC layer. It has therefore been found thatthe α-Al₂O₃ layer of the Examples has a concentration distribution of Sin which the concentration of S decreases in the direction away from thebase-material-side surface of the α-Al₂O₃ layer in the thicknessdirection of the α-Al₂O₃ layer. Moreover, in the Examples, thedifference between Csmax and Csmin is 0.047 to 0.325. At a location at0.5 μm away from the interface between the α-Al₂O₃ layer and the TiClayer toward the inside of the α-Al₂O₃ layer, the grain size of α-Al₂O₃crystal grains is 1.85 μm or less.

The Comparative Examples (Samples A5, A6) for which the content of H₂Sgas was not momentarily increased cannot be evaluated as havingexcellent wear resistance and fracture resistance as seen from Table 4.Moreover, the Comparative Examples fail to have sufficient slidability.

<Analysis>

In the present Examples, H₂S gas with a content of 0.6 vol % wasintroduced and additionally H₂S gas with a predetermined pulse width(0.8 to 1.3 minutes) and a predetermined pulse height (150%) wasintroduced three times to form the α-Al₂O₃ layer. The α-Al₂O₃ layer hada TC(006) of more than 5 and a concentration distribution of S in whichthe concentration of S decreased in the direction away from thesubstrate-side surface of the α-Al₂O₃ layer. The cutting tools of theExamples having a coating including such an α-Al₂O₃ layer exhibited theperformance that Vb was 0.168 or less and the time to fracture was 5.9minutes or more, and therefore had excellent wear resistance andfracture resistance. It was also clarified that the cutting toolexhibited a friction coefficient (μ) of 0.38 or less and therefore hadexcellent slidability as well. Accordingly, the cutting tools of theExamples can achieve an extended life.

Example 2

<Preparation of Base Material>

A base material formed of a cemented carbide base material (manufacturedby Sumitomo Electric Industries, Ltd.) with a shape of CNMG120408identical to that of Example 1 was prepared. Prepared base materialswere grouped into three groups named Sample B1 to Sample B3. For eachgroup, six base materials were prepared. These base materials had acomposition made up of 92.5 wt % of WC, 6.0 wt % of Co, and 1.5 wt % ofNbC. As will be described later herein, Sample B1 is an Example andSample B2 and sample B3 are Comparative Examples.

<Formation of Coating>

The base materials of Sample B1 to Sample B3 were subjected to honingunder the same conditions to those of Example 1 and set in a chemicalvapor deposition apparatus, and a coating was formed on a surface ofeach base material by the CVD method. In Example 2, the TiN layer wasnot included in the structure of the coating.

In the process of forming the α-Al₂O₃ layer by the CVD method, thesurface of the TiCNO layer formed on the surface of the TiCN layer wasoxidized to cause nucleation of α-Al₂O₃ crystal grains, and subsequentlythe α-Al₂O₃ layer was formed. In particular, for nucleation of α-Al₂O₃crystal grains and subsequent formation of the α-Al₂O₃ layer, thecontent of H₂S gas in the raw material gas to be introduced was set to1.7 vol %. The content of each gas in the composition including the H₂Sgas of the raw material gas is indicated in Table 5 below.

It should be noted that, for nucleation of α-Al₂O₃ crystal grains, thecontent of H₂S gas which was originally set to 1.7 vol % was momentarilymade higher than 1.7 vol % and this H₂S gas was introduced. After this,the content of H₂S gas was set to 1.7 vol % and the raw material gas wasused to grow α-Al₂O₃ crystal grains and form the α-Al₂O₃ layer.Respective contents of the constituent gases including H₂S in thecomposition of the raw material gas are indicated in Table 5 below.

Particularly in Example 2, the period of one cycle (pulse period) inwhich the content of introduced H₂S gas was momentarily increased wasvaried. Specifically, the pulse period of Sample B1 was three minutes,the pulse period of Sample B2 was seven minutes, and the pulse period ofSample B3 was one minute. Moreover, commonly to Sample B1 to Sample B3,the pulse height was set to 130% (namely 2.21 vol %) relative to thecontent 1.7 vol % of H₂S gas, the pulse width was set to two minutes,and H₂S gas with its content momentarily increased was introduced twice.These conditions are indicated in the following Table 5.

TABLE 5 pulse height temperature pressure normal and pulse pulse Samplein furnace in furnace H₂ CO CO₂ O₂ AlCl₃ HCl H₂S number width period No.(° C.) (hPa) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %) (vol %) oftimes (min) (min) Example B1 980 85 remainder 3 1.5 0.007 1.5 4 1.7130%, 2.0 3 twice Comparative B2 980 85 remainder 3 1.5 0.007 1.5 4 1.7130%, 2.0 7 Example twice B3 980 85 remainder 3 1.5 0.007 1.5 4 1.7130%, 2.0 1 twice

The layer structure of the coating formed for each of Sample B1 toSample B3 is made up of a TiN layer, a TiCN layer, a TiCNO layer, anα-Al₂O₃ layer, and a TiC layer in order from the base material. In thefollowing Table 6, the layer structure of Sample B1 to Sample B3 and thethickness (μm) of each layer are indicated.

TABLE 6 Sample No. layer structure and layer thickness (μm) Example B1base material/TiN(0.3)/TiCN(9.8)/TiCNO(1.0)/ Al2O3(6.2)/TiC(0.3)Comparative B2 base material/TiN(0.3)/TiCN(9.8)/TiCNO(1.0)/ ExampleAl2O3(6.2)/TiC(0.3) B3 base material/TiN(0.2)/TiCN(9.7)/TiCNO(1.1)/Al2O3(6.5)/TiC(0.3)

<Test Details>

In Example 2, as described above, six samples were prepared for each ofSample B1 to Sample B3. For the first sample out of the six samples, theflank face was irradiated with x-ray to measure the TC(006) of theα-Al₂O₃ layer. For the second sample, the S content of the α-Al₂O₃ layerwas measured. For the third sample, the grain size of α-Al₂O₃ crystalgrains was measured. For the fourth sample, the slidability wasevaluated. For the fifth sample, the wear resistance was evaluated. Forthe sixth sample, the fracture resistance was evaluated.

The S content of the α-Al₂O₃ layer was measured at measurement points(first measurement point to fifth measurement point). The measurementpoint were set at intervals of 1 μm from the position directly on theinterface between the α-Al₂O₃ layer and the TiCNO layer adjacent to thebase material side surface of the α-Al₂O₃ layer, toward the surface ofthe coating (see FIG. 1). The grain size of α-Al₂O₃ crystal grains wasmeasured at 0.5 μm away from the interface between the α-Al₂O₃ layer andthe TiC layer adjacent to the α-Al₂O₃ layer oppositely to the basematerial.

The methods of evaluating the wear resistance, the fracture resistance,and the slidability are as follows. The results of the evaluation areindicated in Table 7 below.

<Evaluation of Wear Resistance>

Workpiece: FCD700 (JIS)

Cutting Speed: 120 m/min

Feed: 0.3 mm/rev

Depth of Cut: 2.0 mm

Cutting Oil: wet

Cutting Time: 10 min

Evaluation: The wear width Vb (mm) of the flank face after cutting wasperformed for 10 minutes was measured.

The wear resistance is evaluated as follows. A cutting tool is set on anNC lathe, cutting of a workpiece is performed with the cutting tool fora predetermined time. After this, a wear width (Vb) formed on the flankface of the cutting tool is observed to evaluate the wear resistance. Acutting tool with a smaller value of the wear width (Vb) can beevaluated as being higher in wear resistance.

<Evaluation of Fracture Resistance>

Workpiece: FCD450 (JIS), grooved material

Cutting Speed: 250 m/min

Feed: 0.2 mm/rev

Depth of Cut: 1.5 mm

Cutting Oil: wet

Evaluation: The elapsed time (minutes) before chipping or fracture wasmeasured.

The fracture resistance is evaluated as follows. A cutting tool is seton an NC lathe, cutting of a workpiece is performed with the cuttingtool, and the elapsed time before chipping or fracture occurs to thecutting tool is measured to evaluate the fracture resistance. Therefore,a cutting tool with a longer elapsed time before chipping or fracturecan be evaluated as being higher in fracture resistance.

<Evaluation of Slidability>

The slidability was evaluated by measuring the friction coefficient (μ).The friction coefficient (μ) was measured by the pin-on-disk methodunder conditions of a 10 N load and room temperature. A cutting toolwith a smaller value of the friction coefficient (μ) has highersmoothness and can be evaluated as being higher in slidability.

TABLE 7 slida- S content (at. %) bility cutting performance 1st 2nd 3rd4th 5th fric- wear time mea- mea- mea- mea- mea- tion resis- to XRDsure- sure- sure- sure- sure- coeffi- grain tance frac- perfor- Sampleresult ment ment ment ment ment Csmax Csmax − cient size Vb ture manceNo. TC(006) point point point point point (at. %) Csmin μ μm (mm) (min)rating Example B1 7.38 0.850 0.087 0.067 0.009 0.008 0.850 0.842 0.390.83 0.156 4.2 B Comparative B2 3.87 0.003 0.003 0.002 0.002 0.002 0.0030.001 0.58 2.11 0.178 2.1 C Example B3 4.15 1.205 0.150 0.023 0.0090.008 1.205 1.197 0.92 3.18 0.212 1.6 D

The rating of evaluation represented for example by symbol AA in Table 7is defined as follows.

AA: highly excellent in wear resistance, fracture resistance, andslidability (Vb of 0.165 or less, time to fracture of 6 minutes or more,and fracture coefficient (μ) of 0.35 or less)

A: excellent in wear resistance, fracture resistance, and slidability(two of the requirements, namely Vb of 0.165 or less, time to fractureof 6 minutes or more, and fracture coefficient (μ) of 0.35 or less aresatisfied)

B: sufficient in required wear resistance, fracture resistance, andslidability (Vb of 0.165 to 0.170, time to fracture of 4 to 6 minutes,and fracture coefficient (μ) of 0.35 to 0.54)

C: insufficient in wear resistance, fracture resistance, and slidability(Vb of 0.170 to 0.180 or time to fracture of 4 to 6 minutes, andfracture coefficient (μ) of more than 0.54)

D: impossible (evaluation is impossible due to occurrence of coarsegrains)

<Results of Evaluation>

As seen from Table 7, the Example corresponding to Sample B1 exhibitsthe performance that Vb (mm) is 0.156 and the performance that the timeto fracture is 4.2 minutes and thus can be evaluated as having excellentwear resistance and fracture resistance. As to the evaluation ofslidability, the friction coefficient (μ) of the Example is 0.39, and ithas been made clear that the Example with a friction coefficient of 0.39has sufficient slidability.

As to the XRD results in Table 7, the Example has a TC(006) of theα-Al₂O₃ layer of more than 5. As to the S content, a Csmax of 0.0850 at.% is obtained at the first measurement point included in the α-Al₂O₃layer and located at a side of the TiCNO layer. In the thicknessdirection of the α-Al₂O₃ layer, the S content decreases in the directionaway from the TiCNO layer, and a Csmin of 0.008 at. % is obtained at thefifth measurement point included in the α-Al₂O₃ layer and located at aside of the TiC layer. It has therefore been found that the α-Al₂O₃layer of the Example has a concentration distribution of S in which theconcentration of S decreases in the direction away from thebase-material-side surface of the α-Al₂O₃ layer in the thicknessdirection of the α-Al₂O₃ layer. Moreover, in the Example, the differencebetween Csmax and Csmin is 0.842. At a location at 0.5 μm away from theinterface between the α-Al₂O₃ layer and the TiC layer toward the insideof the α-Al₂O₃ layer, the grain size of α-Al₂O₃ crystal grains is 0.83μm.

<Analysis>

In the present Example, H₂S gas with a content of 1.7 vol % wasintroduced and additionally H₂S gas with its content increasedmomentarily for each predetermined pulse period (3 minutes) wasintroduced twice to form the α-Al₂O₃ layer. The α-Al₂O₃ layer had aTC(006) of more than 5 and a concentration distribution of S in whichthe concentration of S decreased in the direction away from thesubstrate-side surface of the α-Al₂O₃ layer. It was found that thecutting tools of the Example having a coating including such an α-Al₂O₃layer exhibited the performance that Vb was 0.156 and the time tofracture was 4.2 minutes, and was therefore sufficient in necessary wearresistance and fracture resistance. It was also clarified that thecutting tool exhibited a friction coefficient (μ) of 0.39 and thereforehad excellent slidability as well. Accordingly, the cutting tool of theExample can achieve an extended life.

As clearly seen from Tables 4 and 7 that the surface-coated cuttingtools of the Examples are excellent in wear resistance, fractureresistance, and slidability of the cutting edge. Accordingly, eachExample is considered as being excellent as compared with eachComparative Example and as being capable of achieving an extended life.

While the embodiment and examples of the present invention have beendescribed above, it is originally intended that the above-describedfeatures of the embodiment and examples may be combined as appropriateor modified in various manners.

It should be construed that the embodiment disclosed herein is given byway of example in all respects, not by way of limitation. It is intendedthat the scope of the present invention is defined by claims, not by theabove-described embodiment, and encompasses all modifications equivalentin meaning and scope to the claims.

REFERENCE SIGNS LIST

1 α-Al₂O₃ layer; 2 TiCN layer; 3 TiCNO layer; 4 measurement point; 41first measurement point; 42 second measurement point; 43 thirdmeasurement point; 44 fourth measurement point; 45 fifth measurementpoint

1. A surface-coated cutting tool comprising a base material and acoating formed on the base material, the coating including an α-Al₂O₃layer, the α-Al₂O₃ layer containing a plurality of α-Al₂O₃ crystalgrains and sulfur, and having a TC(006) of more than 5 in texturecoefficient TC(hkl), and the sulfur having a concentration distributionin which a concentration of the sulfur decreases in a direction awayfrom a base-material-side surface of the α-Al₂O₃ layer, in a thicknessdirection of the α-Al₂O₃ layer.
 2. The surface-coated cutting toolaccording to claim 1, wherein the α-Al₂O₃ crystal grains having a grainsize of 0.2 to 2 μm occupy 20 to 80% by area of a measurement surface,the measurement surface is in parallel with a surface of the α-Al₂O₃layer or parallel with an interface between the α-Al₂O₃ layer and anadjacent layer, the adjacent layer is adjacent to the α-Al₂O₃ layer andlocated on an opposite side to the base material, and the measurementsurface is located at a depth of 0.5 μm from the surface of the α-Al₂O₃layer or the interface.
 3. The surface-coated cutting tool according toclaim 1, wherein the TC(006) is more than
 6. 4. The surface-coatedcutting tool according to claim 1, wherein the TC(006) is more than 7.5. The surface-coated cutting tool according to claim 1, wherein amaximum concentration Csmax of the sulfur in the concentrationdistribution appears in a region of 1 μm from an interface between theα-Al₂O₃ layer and the base material, or from an interface between theα-Al₂O₃ layer and a layer adjacent to the α-Al₂O₃ layer and located onthe same side as the base material, in the thickness direction of theα-Al₂O₃ layer, and a minimum concentration Csmin of the sulfur in theconcentration distribution appears in a region of 1 μm from a surface ofthe α-Al₂O₃ layer or from an interface between the α-Al₂O₃ layer and alayer adjacent to the α-Al₂O₃ layer and located on an opposite side tothe base material, in the thickness direction of the α-Al₂O₃ layer, andthe Csmax is 0.005 to 1 at. %, the Csmin is 0.001 to 0.1 at. %, and theCsmax and the Csmin meet a relation Csmax>Csmin.
 6. The surface-coatedcutting tool according to claim 1, wherein a maximum concentration Csmaxof the sulfur in the concentration distribution is 0.005 to 1 at. %. 7.The surface-coated cutting tool according to claim 1, wherein theα-Al₂O₃ layer has an average layer thickness of 1 to 15 μm.
 8. Thesurface-coated cutting tool according to claim 1, wherein in a surfaceof the coating, an outermost surface layer in which any one of Ticarbide, Ti nitride, and Ti boride is a main component is disposed. 9.The surface-coated cutting tool according to claim 1, wherein thecoating has an intermediate layer between the α-Al₂O₃ layer and the basematerial, the intermediate layer contains acicular TiCNO or acicularTiBN and has an average layer thickness of 0.3 to 1 μm, and a differencebetween a maximum thickness and a minimum thickness of the intermediatelayer is 0.3 μm or more.
 10. A method of manufacturing a surface-coatedcutting tool as recited in claim 1, the method comprising the step offorming, on the base material by a CVD method, the coating including theα-Al₂O₃ layer, in the step, a content of H2S gas contained in a rawmaterial gas in an initial stage of formation of the α-Al₂O₃ layer being0.5 to 5 vol %, and the content of H2S gas being momentarily increasedto 0.65 to 7 vol %.